1. Field of the Invention
The present invention is related to an image forming apparatus such as a copying machine, printer, facsimile machine, and combined machine of them using electrophotographic process, specifically to an image forming apparatus having a pair of transfer rollers consisting of a rigid roller and an elastic roller pinching the copy sheet between them for transferring it.
2. Description of the Related Art
In a copying machine, printer, facsimile machine, or combined machine of them using electrophotographic process, high accuracy is required for the feeding velocity of a copy sheet transferred by means of transfer rollers, for the feeding velocity of the copy sheet influences directly the magnification, etc. of the image to be transferred to the copy sheet from the image carrying body such as a photoreceptor.
Particularly, high accuracy is required to a pair of registration rollers for adjusting the position of the copy sheet fed from the paper feeder cassette and allow the sheet to advance to the image transfer position synchronizing with the timing of image formation by carrying body such as a photoreceptor, since the feed velocity of the copy sheet directly influences the quality of image. There has been adopted a method, in which an elastic roller having a rubber surface of hardness of about, for example, JIS-A 65°˜90° on its periphery is pressed to a rigid roller made of metal or having resin material on its periphery, the rollers are connected by means of gear wheels to transmit driving force from one of the roller to the other, and the copy sheet is pinched between the rollers in order to achieve the feeding of copy sheet with stable velocity and allowance for the variation in feeding load.
In this case, as the rigid roller and elastic roller are connected with gear wheels, the diameter of each roller is determined so that the peripheral velocities of both the rollers coincide with each other in consideration of the gear ratio. The diameters of this state are hereafter referred to as equal-velocity diameters. If the diameters deviate from the equal-velocity diameters, there occurs a slip between the rollers. However, there is inevitably variation in the diameter of each roller due to machining tolerance of diameter. Particularly, the tolerance of the diameter of the elastic roller having rubber material on its periphery is larger than that of the rigid roller owing to machining condition. Therefore, there has been a problem that the feeding velocity of copy sheet is largely influenced by the diameter of the elastic roller and feeding of the copy sheet with stable velocity can not be secured.
For feeding the copy sheet accurately with stable velocity, an image forming apparatus used for copying images onto a variety of kinds and sizes of copy sheets is disclosed, for example, in Japanese Laid-Open Patent Application No. 2001-166607 (hereafter referred to as Patent literature 1). An image forming apparatus had been disclosed, in which a pair of preliminary feed rollers was additionally provided between the registration rollers and the image transfer position with a sensor for detecting the leading edge of the copy sheet provided downstream of the registration rollers before the pair of preliminary feed rollers, because it was difficult for a pair of registration rollers, which consists of a driving roller and a follower roller and feeds the copy sheet to the image transfer position in synchronization with the timing of toner image formation on an image carrier device such as a photoreceptor, to feed various kinds of copy sheets with the accuracy of position required. The patent literature 1 teaches that even with the mechanism described above it became difficult to attain required accuracy. To deal with this and with the variation in copy sheet transfer performance due to the wear of the transfer rollers by the use over a long period of time, the patent literature 1 proposed to divide the guide member for guiding the copy sheet to the image transfer position into two, each for the pair of registration rollers and for the pair of preliminary feed rollers, the relative position of these guide members being able to be adjusted, and to compose the driving roller of the pair of preliminary feed rollers with wear-resistant material.
Further, in Japanese Laid-Open Patent Application No. 2000-351470 (hereafter referred to as Patent literature 2) is disclosed an image forming apparatus, in which the transfer rollers of an ink-jet printer is composed of a roller having hard material on the periphery thereof and a roller having material of high friction coefficient such as rubber on the periphery thereof, the latter rubber roller is divided in the direction of axis thereof such that guide rollers each having a diameter smaller than that of the central roller and hardness harder than that of the central roller are provided to the both side of the central roller in order to keep the accuracy of copy sheet transfer irrespective of the thickness and size of the sheet. With the configuration of the rubber roller like this, the both side parts of the rubber roller are prevented from being excessively deformed by virtue of the guide rollers, which prevents the sheet from advancing obliquely when a sheet large in thickness but small in width is fed.
Further, in Japanese Patent No. 3140152 (hereafter referred to as Patent literature 3) is disclosed an image forming apparatus, in which sheet feeing mechanism is composed so that the transfer distance per step of the stepping motor of each pair of rollers are different in order to prevent the occurrence of resonance of two pairs of rollers which induces noisy occurrence and affects the feeding performance.
However, the sheet transfer mechanism disclosed in patent literature 1 is composed of a driving roller and a follower roller not that both the rollers are connected by gear wheels. Further, as a sensor for detecting the leading edge of copy sheet is provided downstream from a pair of registration rollers and a pair of preliminary feed rollers is provided between said pair of registration rollers and image transfer position, the mechanism is complicated resulting in higher cost. The disclosure of patent literature 2 is an art to prevent oblique advancing of the copy sheet when feeding the sheet which is thick but small in width and can not solve the problem of the fluctuation of sheet feeding velocity due to the variations of the diameters of the rollers. The disclosure of patent literature 3 is an art of preventing mainly noisy occurrence due to the resonance of two pairs of rollers located at different positions and contributes little to increase the accuracy of feeding the copy sheet.
Accordingly, the object of the present invention is to provide a pair of rollers for feeding copy sheet, the pair of rollers being composed of an elastic roller pressed against a rigid roller and driven by means of a driving mechanism to feed the sheet pinching it between the rollers, composed such that the copy sheet can be fed with stable velocity and high accuracy, even if there are variations in diameters of the rollers due to machining tolerance, with simple construction and low cost for an image forming apparatus.